It could be the most spectacular exhibition of the year and the National Gallery's most popular blockbuster. So many visitors are expected that the National Gallery will dispense with its basement space in the Sainsbury Wing to accommodate the first British retrospective of the great Spanish master Diego Velaquez.

Instead of the galleries in the basement, which proved squashed and bad-tempered for the 267,939 visitors who attended the Titian exhibition in 2003, the exhibition of 40 artworks by the painter, who lived from 1599-1660, will luxuriate in four of the museum's main galleries. "It will be an exceptional exhibition," said the National Gallery's director, Charles Saumarez Smith. Depending on demand, it is expected that up to 300,000 people could visit the show, making it most popular paying exhibition ever at the art gallery (with full-price tickets at &pound;12). Seeing Salvation, a free show based on the image of Christ, was seen by 355,000 in 2000.

Meanwhile, 19th-century oil paintings that normally occupy the four main galleries, including Van Gogh's Sunflowers and Seurat's Bathers, will be shunted out and partially rehung in the Sainsbury Wing's basement space.

The show will include 40 oil paintings, nearly half of the 80 or so that are extant. There are 18 Velazquezes in the UK, 16 of which will be brought together for the exhibition, which opens in September. Its curator, the National Gallery's Dawson Carr, called Velaquez "the greatest painter ... He has an uncanny ability to convey a sense of truth. He goes beyond realism to create a sense of psychological presence. We believe we are standing before these people he paints."

But the exhibition will be without Velaquez's two most famous artworks: Las Meninas, his enigmatic painting of the Infanta Margarita with the king and queen reflected in a mirror on the far wall; and Pope Innocent X from Rome's Palazzo Doria Pamphilj, one of the greatest portraits ever made.

Of Las Meninas, which is at the Prado museum in Madrid, Mr Carr said: "That oil painting has only left Madridonce - during the Spanish civil war, when it was sent to Valenciafor safety. It's not going anywhere: nor should it ... If we had asked for Las Meninas, they wouldn't have taken us seriously."

Mr Saumarez Smith said: "Prince Jonathan Doria Pamphilj is a great friend of this institution. But that painting [Pope Innocent X], has only left Romeonce, when the palace was closed for renovations 10 years ago."

Mr Carr said the exhibition had been made possible only through the cooperation of the Prado, the major holder of Velaquezes from the Spanish royal collection. It will lend the National Gallery eight of its oil paintings by the artist. These include the outstanding works Apollo at the Forge of Vulcan, Philip IV as a Hunter, and the portrait of the court dwarf Francisco Lezcano.

But Mr Carr said not everything on his wishlist had been granted. "We wanted a Mars from the 1630s, a Venus from the 1640s, and Mercury and Argus from 1659," he said. "Nevertheless, eight is an extraordinary number." The vast majority of the artworks that will be lent from overseas had never been seen in the UKbefore, he added.

The National Gallery's nine Velazquezes are the biggest single collection of works by the painter outside Spain. They are mostly early artworks, made before he rose to dizzy heights at the court of Philip IV. By bringing in artworks from abroad, said Mr Carr, the sweep of Velazquez's career would be fully appreciated, and the development of his "marvellous technique", which is "really about suggestion rather than elaboration" with "details that, close up, dissolve into dabs and dashes of paint". The beautiful early artworks, such as Kitchen Scene with Christ in the House of Martha and Mary, in the National Gallery, still struggle with depth and perspective. By the late oil paintings, such as Infanta Margarita in Blue, he was working the paint with effortless virtuosity, using a thin layer and incorporating the brown shade of the canvas to create the texture of a rich, blue-velvet dress.

The National Gallery will also borrow four artworks from Apsley House, London, including an artwork related to the Innocent X portrait, possibly a sketch for it, which is being cleaned. They were captured from the French army by the Duke of Wellington, who was offered them by the king of Spainas a gift of thanks.